1. Summary of the Invention
This invention relates to air-cooled vacuum steam condensers and the isolation of noncondensible gas removal from tube rows, bundles and fan cells by the use of orifices and steam nozzles that isolate the operation of bundles and individual fan cells thereby promoting freeze protection and allowing reverse ambient air flow through the bundles by reversing fan motor rotation that creates a desired protective hot-air recirculation environment.
2. Description of the Background Art
Various air-cooled vacuum steam condensers are disclosed in the patent literature which feature many variations of heat-exchange bundle designs. Few extend their invention into that portion of the steam condensing system that starts with the bundle rear headers and terminates with the gas/air removal equipment. Without exception, they all show an evacuation piping system that connects the bundle rear headers direct to a common manifold. This manifold pipe then leads to the gas/air removal equipment commonly known as the Steam-Jet-Air-Ejector (SJAE). Air-cooled steam condenser patents featuring this direct connected common manifold evacuation system for their bundle rear headers are disclosed in the patent literature.
Typical of overall air-cooled vacuum systems of this type are those disclosed in my recent patents, U.S. Pat. Nos. 4,903,491; 4,905,474 and 4,926,931, the subject matter of which is incorporated herein by reference. Other air-cooled vacuum systems, or at least parts thereof, are disclosed in my earlier U.S. Pat. Nos. 3,968,834; 4,129,180 and 4,518,035. Patents disclosing full air-cooled vacuum systems, or at least significant parts thereof, include U.S. Pat. Nos. 2,217,410 and 2,247,056 to Howard; 3,289,742 to Niemann; 4,168,742 to Kluppel; 4,190,102 to Gerz and 4,417,619 to Minami. Lastly, portions of air-cooled vacuum systems which do not disclose the specifics of the full recirculation of the steam and the movement of the non-condensible gasses include U.S. Pat. Nos. 3,543,843 to Gunter; 3,556,204 to Dehne; 3,677,338 to Staub; 3,705,621 to Schoonman; 3,707,185 to Modine as well as 3,887,002 to Schulman.
The object of this invention is to improve the freeze protection features of known air-cooled vacuum steam condensers by a new design of noncondensible gas manifolding and removal system. The proposed improvements are in that portion of the system which starts at the bundle rear headers and ends with the Steam-Jet-Air-Ejector (SJAE) equipment.
There are many reasons for air-cooled steam condensers freezing such as bundle design, system design, controls, operation and uncontrolled external/internal influences. This condenser improvement deals with the uncontrolled external/internal influences that are the cause of many unexplained tube freezing problems. It presents a relatively simple solution to this complex problem which is applicable to all vacuum steam condensers.
Experience has shown that bundles and tubes of certain fan cells in large steam condenser installations are more prone to freeze than the bundles and tubes of other fan cells in the same condenser. The question that arises is why should this happen when all the steam condensing tubes/bundles/fan cells in a given condenser tower are of identical construction. The simple answer is that every tube/bundle/fan cell is subject to different external/internal influences that affect its thermal performance.
Some of the major external influences that affect the thermal performance of each tube/bundle/fan cell differently are wind effects, natural draft, hot-air recirculation, wind walls, air flow restrictions, structure shielding and clogged fins. For example, some fan cells are protected from cold winds by being located immediately behind a large wind wall while other fan cells are in the direct path of the cold blast.
Internal influences affecting fan cell performances are steam duct length, size, elbows, straightening vanes, steam velocities, tees and valves. For example, some fan cells are located close to the turbine exhaust while others are in the furthest reaches of the structure. The same applies to the noncondensible gas removal equipment (SJAE) and its physical location relative to the far-out fan cell it is serving. Another most important influence concerns differences in mass air flow delivered by a mechanical draft fan as a result of differences in blade profile, pitch setting and motor speed.
All of the factors listed above affect the quantity of steam condensed by each and every tube/bundle/fan cell. This by itself would not be cause for concern if it were not for the fact that because the bundles condense different quantities of steam they have different steam pressure-drops across their tubes. This results in different rear header pressures and this is where the problem lies. Since all of the rear headers are connected to the same noncondensible gas-piping manifold system, there is a backflow exchange of gas/vapor mixtures through this common piping amongst the rear headers. In this backflow process the steam pressures quickly equalize by the formation of stagnant gas pockets of varying lengths inside the steam condensing tubes. The system is self-compensating in this process but it does so at the expense of creating stagnant gas pockets inside the finned tubes. These gas pockets are cold because they lack steam and any condensate flowing through them can freeze. Tube rupture generally follows condensate freezing. When the ambient air temperature is above 32 F., the gas pockets blanket heat transfer surfaces which then lowers the steam condensing capability of the unit.
The uncontrolled external/internal influences affect each tube/bundle/fan cell differently because of their physical location in the condenser installation. An uncontrollable fluid disruption in one corner of the condenser automatically causes a fluid disruption in the remainder of the condenser. Trying to eliminate or neutralize these influences is an impossible task as there are just too many of them. Most are beyond the control of the condenser designer. The approach to this problem is not to try to solve or attack the individual influences but to stop and prevent the backflow interchange of noncondensible gases and vapors amongst the rear headers through their common manifold piping system.
The obvious solution to the problem is to install check valves in the manifold piping system. Fluid backflow in normal piping systems is prevented by the installation of check valves. Unfortunately, check valves cannot be used in the noncondensible gas evacuation system of air-cooled vacuum steam condensers because of the extremely low fluid pressures in the system which sometimes measure a fraction of an inch of mercury. There is no commercially available check valve that could operate under those conditions. To circumvent this problem, an indirect approach is used. Instead of using pipe check valves as in the direct approach, flow devices are selected that perform other necesary functions and also act as one-way valves. Orifices are used primarily to control fluid flow rate but they also act as one-way valves for fluids flowing from a higher pressure to a lower pressure. Steam ejectors are vacuum producing devices but they too act as one-way valves once the suction gases mix with the high pressure motive steam in the nozzle. Hence the devices selected for use in the new noncondensible gas evacuating system are orifices and steam ejectors. The orifices are used to isolate bundle tube rows and the steam ejectors to isolate fan cells. The old concept of connecting all the bundle rear headers directly to a common manifold piping system that leads to the suction side of a large steam ejector is discarded. A common manifold piping system is still used but it cannot backflow the gas/vapors.
The new gas/vapor withdrawal system consisting of orifices discharging to their steam ejector offers three (3) basic degrees of ISOLATION from backflow to the bundles and their fan cell. The greater the isolation, the more costly the design. The highest degree of isolation (No. 1) is obtaining by using a flow control orifice in each row of the bundle with final gas/vapor flow terminating in an individual steam ejector. There is absolutely no backflow amongst the rear headers or bundles of this design. The next highest degree of isolation (No. 2) is the same as No. 1 except that the one steam ejector is shared by the other bundles in that particular fan cell. In this design one of the four bundles may experience an air flow disturbance that would imbalance the gas/vapor flow rate leaving its orifice. There may or may not be any backflow or fluid interchange but there might be a smaller evacuation rate than would be desired. The third degree of isolation (No. 3) has no orifices installed in the individual rear headers. Instead, one orifice is installed in each of the collective tube rows of the group of bundles representing one fan cell. The final gas/vapor discharge is to a steam ejector which serves all the bundles of a single fan cell. In this design there may be some backflow between the same tube row of different bundles but not between different tube rows.
This new rear-end condenser design isolates the bundles and their fan cells such that they become small independent steam condensers. They react to external/internal influences in their own manner without affecting the performance of other bundles or fan cells. The fans can be individually operated to deliver varying quantities of cooling air without upsetting the gas/vapor withdrawal process.
The air flow direction through the bundles can also be reversed without upsetting the gas/vapor withdrawal process inside the bundle rear headers by simply interchanging orifices serving tube row No. 1 with tube row No. 4 and tube row No. 3 and tube row No. 2. This orifice reversal can either be performed manually or automatically with the use of electric solenoid valves. Reverse air flow is accomplished by reversing the fan rotation or changing the fan pitch. Reverse air flow in all or some of the fan cells allows the entire condenser environment to be bathed in warm recirculated air. This type of operation eases the freeze danger to the condenser and surrounding plant equipment.
A new air-cooled intercondenser replacing the conventional water-cooled intercondenser for the Steam-Jet-Air-Ejector (SJAE) set is revealed. Since a piping manifold is required to connect all the steam ejector discharges that are located at each fan cell, it can be made to serve a double purpose. By using finned tubes instead of conventional pipe, the manifold can serve as both a fluid conduit and a heat exchanger to condense the steam inside. This eliminates the need for the conventional costly water-cooled intercondenser.
Accordingly, it is an object of the present invention to provide an improved multi fan-cell system for condensing steam vapors contaminated with air and gases, the system being isolated into single fan cells for the removal and discharge of its noncondensible gas/vapors, each fan cell having one fan and a predetermined number of steam condensing bundles which includes a front header, finned tubes and rear header means whose gas/vapors are extracted and isolated from all other fan cells by a vacuum-producing steam ejector means coupled to the bundle rear headers by pipe and manifold means all located and installed within the confines of the fan cell they serve.
It is a further object of the invention to effectively isolate the output of each rear header pipe and its associated rear header from all other rear header pipes of a different tube row number and their associated rear headers.
It is a further object of the invention to allow reverse air-flow through the bundles by reversing the direction of rotation of fan blades for selected fan cells causing the hot air to be recirculated and discharged downward toward grade elevation thereby raising the ambient air temperature flowing through adjacent fan cells.
It is a further object of the invention to vary the sizes of orifices in pipes from rear headers as a function of the direction of air flow throughout the bundles and rotation of the fans.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or by modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.